To make a last backup, unpair the Watch with your old phone first. Next, install your new iPhone and leave it close to your Watch. Your new phone would suggest to you to pair; select the option of restoring with the latest Watch backup and proceed with the steps.
To pair your Apple Watch with a new iPhone, first unpair it from your old phone. Then, turn on your new iPhone and open the Apple Watch app. Follow the on-screen instructions to pair and restore your backup. Make sure both devices are close and connected to Wi-Fi.
